In recent years, electrically conductive resins have been developed and such conductive resins are increasingly used to make electrical connections between metal conductors, depending on the application involved and the location of use. For example, connecting structures of this type are employed in the detecting parts of oil leakage detection sensors which are used in order to check for oil leakage from oil tanks, piping in chemical plants, or aqueous or organic fluid carrying piplines.
Conductive resins such as carbon-filled polytetrafluoroethylene (PTFE) endowed with electrical conductivity by the admixture of powdered carbon in a PTFE insulation material, are used in the sensing parts of oil leakage detection sensors. PTFE resins have the property of repelling water while allowing the permeation of oil. In addition, carbon-filled PTFE resins have the property of showing a change in electrical resistance when permeated by oil. Oil leakage detecting sensors utilize this property of carbon-filled PTFE resins.
A conventional oil leakage detecting sensor has a pair of conductive metal lead wires which are installed so that they are separated from each other. In the detecting part of the sensor, these lead wires are electrically connected with each other by a resistor or connecting piece which consists of carbon-filled PTFE resin. This resistor is usually in the form of a band between the wires. The respective edges of the resistor are bonded to the circumferential surfaces of the lead wires by means of a conductive adhesive, with each edge covering approximately half of the circumferential surface of the corresponding lead wire. Furthermore, these lead wires and the resistor are covered by an insulating substrate which may comprise a PTFE resin.
The respective lead wires of the oil leakage detecting sensor are connected to a detection circuit, so that a closed circuit is formed by the lead wires, resistor, and detection circuit. When oil contacts the detecting part of the oil leakage detecting sensor, the oil permeates through the insulating substrate and penetrates into the resistor. As a result, the electrical resistance of the resistor changes, so that an accompanying electrical change occurs in the detection circuit. As a result, the leakage of oil can be detected.
In the above conventional connecting structure, different materials, such as the lead wires, which are made of metal, and a resistor, which is made of a resin material, are directly bonded by means of an adhesive. Accordingly, the bonding strength is weak, which may lead to improper or inconsistent contact, and in some cases to an interruption of electrical continuity owing to peeling of the bonded area. As a result, such conventional structures have lacked reliability.
Further, since the resistor is in contact with only about half of the circumferential surface of each of the lead wires, the area of contact is small, so that there is a great increase in electrical resistance in the connecting part. In the case of an oil leakage detecting sensor, such as described above, this leads to a drop in sensitivity.